MONDAY, 25 NOVEMBER 2013
What is sleep? All organisms have a circadian rhythm, a kind of day-night cycle, which allows them to anticipate and prepare for regular changes in their environment. For example, plants will naturally increase production of insect repellent chemicals during the day, when insects are most likely to attack and bacteria activate different genes at different times of day. But sleep is more than just a daily cycle of activity; it is a complex state of inactivity that requires some kind of nervous system. During sleep the organism is less able to react to stimuli and, unlike a coma, this state is quickly reversible.
There is undeniably something special about sleep. We may abstain from eating and drinking for long periods of time, or even indefinitely refrain from reproduction, but even a priest cannot abstain from slumber. So surely there must be some incredibly important reason for sleep, something that makes it fundamental for our survival?
The best answers we have to these questions come, as so often in biology, from studying other animals. If sleep was not essential we ought to be able to identify animals that do not sleep, do not need to compensate for sleep losses, or can lose sleep without any negative consequences. So far, the only animal thought not to sleep is the bullfrog, however this claim has been challenged due to poor experimental design in the original 1967 study. Other research has shown that even ‘lower’ animals like flies and worms do indeed sleep. Dolphins are a particularly interesting case, as they are capable of having only half their brain asleep at a time, a state characterised by slow ‘sleep wave’ activity in that half. Unlike the automated human response, dolphin breathing ceases under anaesthesia, suggesting it is a conscious procedure; this may explain why dolphins have to always retain significant degrees of brain activity when sleeping.
If no animals have been demonstrated to go without sleep altogether, are there any which do not compensate for sleep losses? It was once thought this was the case for honeybees, which do not increase sleep length following sleep deprivation. However, it was later discovered that the duration of slow wave electrical activity in the brain, which is indicative of sleep, increased during recovery. Thus honeybees recover from sleep deprivation by improving sleep quality but not quantity. Interestingly, when pigeons were prevented from sleeping for 10 days, they demonstrated no change in recovery sleep but showed an increase in slow wave brain activity during wakeful periods.
Thus so far we have found no animals which do not sleep at all, or do not compensate for sleep loss. Are there then any animals which can lose sleep without negative consequences? Once again, none have been found so far, and loss of sleep has been shown to lead to cognitive impairment in animals as diverse as flies, humans and birds. So throughout the animal kingdom, from tigers to domestic kittens, a certain amount of sleep, or sleep-like state, appears to be essential for the correct functioning of an organism.
Sleep must surely serve some purpose—if it really were just a wasted eight hours a day it would undoubtedly have been rooted out by evolution long ago. Many theories have been proposed but none has proven definitive. A reduction in metabolic rate has been suggested; sleep allows heart rate to drop by approximately 10 beats per minute and core body temperature to decrease by approximately one degree in humans. Memory consolidation is often also touted as a reason for sleep, although this process may also occur whilst awake. Although no complete explanation has ever been offered as to why sleep is necessary for memory consolidation, recent studies have shown that the brain activity of a mouse allowed to solve a maze during the day is repeated at night. A simpler explanation is that sleep simply allows an organism to keep out of harm’s way, when nothing of great significance is likely to be achieved. Perhaps sleep gives the brain a chance to recover, for example by replenishment of chemicals such as neurotransmitters that carry signals between neurons. We may never know the original purpose of sleep, although it has certainly evolved to be multi-functional.
Whatever the origins of sleep, we undoubtedly need it now. The exact amount required by an individual is, like so many aspects of human biology, incredibly variable. However, there are definite trends over a lifetime, with a gradual decline in the amount of sleep an individual needs, from almost 18 hours a day as a newborn, to around seven hours as an adult. These requirements are important, for example individuals awake for 17 consecutive hours have reduced cognitive performance akin to that produced by drinking two glasses of wine. Night shift workers who regularly have disrupted daytime sleep have also been shown to have increased risk of cardiovascular diseases.
Much of the current sleep research focuses on how we can improve sleep quality. There are four distinct stages within sleep, forming a cycle of about 90 minutes, repeated several times a night. These stages are perhaps more familiar to us than we realise: wake up during the wrong part of your sleep cycle and you often feel groggier than before you went to sleep. The four stages are: rapid eye movement (REM) and stages 1, 2 and 3. REM is the most similar to an awakened state while stage 1 appears to serve mainly as a transition to later stages. Stage 2 has been implicated in memory consolidation, whilst stage 3 is generally considered the most beneficial stage and exhibits the slow wave activity characteristic of deep sleep. Through the night, each cycle gradually includes less stage 3 and more REM, suggesting that we gradually meet our requirement for stage 3 sleep. Given all this information, one radical new approach to sleep research is attempting to cut out the middle man and head straight for deeper sleep, with all its associated benefits. This could drastically reduce the amount of sleep a person needs, something we would all find useful on occasion—that last minute deadline for one.
The research involves a technique called transcranial magnetic stimulation (TMS) that has been shown to instantly induce a deeper sleep state—at least in those volunteers who were able to get to sleep inside the machine. As slow waves characteristic of deep sleep are involved in consolidating newly learnt tasks, using TMS to create localised slow waves in specific groups of neurons may increase the efficacy of information consolidation and allow a decrease in the amount of sleep we need. However, there are both scientific and ethical controversies surrounding TMS. Firstly, some evidence suggests that stage 2 function may be key for memory consolidation. Furthermore, even if TMS was to work perfectly without damaging side effects, it could divide society; those who could afford the technique would get an extra two waking hours per day. There could be an imbalance in the economy of time as well as money. Maybe then, the ancient Egyptian notion of losing our soul while we sleep might not be so far-fetched after all.
Toby McMaster is a 2nd year undergraduate studying Biological Natural Sciences at Jesus College You spend roughly a third of your life doing something we simply do not understand. The ancient Egyptians believed it allowed the soul, Ba, to fly out of the body. The ancient Greeks thought it allowed the receipt of prophecies, until Aristotle who suggested it allowed digestion. Even now, sleep is arguably the most poorly understood aspect of daily human life.